If you want a low-cost and simple emergency power backup, use your car. The 12V battery in the car will supply the power to an inverter such as this 1100 W unit http://www.dcacpowerinverters.com/. Next, feed the inverter output into a back feed breaker such as the ones shown here http://www.interlockkit.com/.
The back feed breaker setup will allow you to supply power to your whole house in an emergency but always keep in mind to never turn on more devices at one time than what your battery/inverter combination will support(in watt capacity). A 1100w backup would power a refrigerator, laptop, and a few lights at the same time. If more power is needed, look into a 1500w inverter. The key is to manage your power use by very selectively turning on/off critical devices.
If you’re not skilled in-home electrical, consult an electrician for back feed breaker installation. Your backup power will last as long as your car has gas to idle. Force the car to idle at 1000-1200 RPM by placing some type of weight on the gas pedal and spin the alternator. A typical car alternator will output 400-500w spinning at 1200rpm, and reach around 1000w turning 2100+ rpm. If you find your battery voltage dropping during emergency use, speed the car idle up to increase the alternator output, not exceeding 2100rpm. Learn more.
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